This application is based on application No. 098312 filed in Japan on Mar. 30, 2001, the content of which incorporated hereinto by reference.
This invention relates to a method of accurately determining remaining capacity of a discharging battery.
Remaining battery capacity can be calculated by subtracting discharge capacity from charge capacity. For a fully charged battery, discharge capacity is subtracted from the fully charged battery capacity. Discharge capacity is calculated from the integral of the discharge current times the discharge efficiency. Discharge efficiency varies depending on temperature and discharge rate. Discharge efficiency decreases when battery temperature decreases. Moreover, although the rate of discharge increases with high current discharge, discharge efficiency decreases. To accurately calculate remaining battery capacity, prior art methods set discharge efficiency from temperature and discharge rate, and calculate discharge capacity based on the set discharge efficiency.
FIG. 1 shows a prior art method of setting discharge efficiency. The method of this figure divides the discharge rate into a plurality of regions with boundaries determined by a plurality of inflections points. In each region, discharge efficiency is approximated by a first order linear function. The first order linear function is specified by a first constant which sets the slope and a second constant which sets an intercept. Consequently, for a specified temperature in one discharge rate region, two constants are required in memory. In the figure, the discharge rate is divided into five regions. Therefore, 10 constants are required in memory for a specified temperature. Further, to specify discharge efficiency for six different temperatures, 60 constants are required in memory.
In addition, the minimum voltage corresponding to the remaining battery capacity setting for a discharged battery also varies depending on temperature and discharge rate. FIG. 2 shows minimum voltage characteristics corresponding to remaining battery capacity which has dropped to 5%. As shown in this figure, minimum voltage at 5% remaining battery capacity can be correctly determined by specifying temperature and discharge rate. To determine minimum voltage from temperature and discharge rate, discharge rate is divided into a plurality of regions. In each region, minimum voltage is approximated by a first order linear function. This first order linear function is specified by a first constant which sets the slope and a second constant which sets an intercept in the same fashion as for discharge efficiency. Consequently, for a specified temperature in one discharge rate region, two constants are required in memory. Since the discharge rate in the figure is divided into five regions, 10 constants are required in memory for a specified temperature. Further, to specify discharge efficiency for six different temperatures, 60 constants are required in memory.
As shown in FIGS. 1 and 2, a method, which divides discharge rate into a plurality of regions and represents discharge efficiency and minimum voltage by first order linear functions in those regions, requires storage of many constants in memory. In particular, since discharge rate must be divided into many more regions for accurate determination of discharge efficiency and minimum voltage, this method has the drawback that an excessive number of constants must be stored. Further, since the linear approximation method is discontinuous at inflection points, a small change in discharge rate can result in abrupt changes in discharge efficiency and minimum voltage. Discharge efficiency and minimum voltage for a real battery do not change abruptly at any discharge rate. For this reason, a method of determining discharge efficiency and minimum voltage based on a first order linear function has difficulty accurately specifying discharge efficiency and minimum voltage for all discharge rates. Therefore, a linear approximation method has the drawback that remaining battery capacity cannot be accurately calculated.
The present invention was developed to solve the types of problems described above. Thus it is a primary object of the present invention to provide a method of calculating remaining battery capacity which can accurately determine remaining battery capacity while reducing the number of constants stored in memory.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.
The method of calculating remaining battery capacity of the present invention specifies battery discharge efficiency with discharge rate as a parameter, and calculates remaining battery capacity from integrated discharge current based on the specified discharge efficiency. This method stores discharge efficiency as an nth order function of discharge rate where n is 2 or greater, computes discharge efficiency based on the nth order function in memory with discharge rate as a parameter, and calculates remaining battery capacity based on the computed discharge efficiency. In this application, the term parameter is used with a meaning equivalent to independent variable in the 3rd order functions of the specific embodiments below.
The method of calculating remaining capacity can also specify battery discharge efficiency with temperature as a parameter, and calculate remaining battery capacity from integrated discharge current based on the specified discharge efficiency. This method stores discharge efficiency as an nth order function of temperature where n is 2 or greater, computes discharge efficiency based on the nth order function in memory with temperature as a parameter, and calculates remaining battery capacity based on the computed discharge efficiency.
Further, the method of calculating remaining capacity can also specify battery discharge efficiency with temperature and discharge rate as parameters, and calculate remaining battery capacity from integrated discharge current based on the specified discharge efficiency. This method stores discharge efficiency as an nth order function of temperature and discharge rate where n is 2 or greater, computes discharge efficiency based on the nth order function in memory with temperature and discharge rate as parameters, and calculates remaining battery capacity based on the computed discharge efficiency.
In addition, the method of calculating remaining battery capacity of the present invention specifies minimum battery voltage with discharge rate as a parameter, and determines remaining battery capacity by detecting when discharging battery voltage reaches the specified minimum voltage. This method stores minimum voltage as an nth order function of discharge rate where n is 2 or greater, computes minimum voltage based on the nth order function in memory with discharge rate as a parameter, and assumes remaining battery capacity has reached a set capacity when battery voltage drops to the computed minimum voltage.
The method of calculating remaining battery capacity can also specify minimum battery voltage with temperature as a parameter, and determine remaining battery capacity by detecting when discharging battery voltage reaches the specified minimum voltage. This method stores minimum voltage as an nth order function of temperature where n is 2 or greater, computes minimum voltage based on the nth order function in memory with temperature as a parameter, and assumes remaining battery capacity has reached a set capacity when battery voltage drops to the computed minimum voltage.
Finally, the method of calculating remaining battery capacity can also specify minimum battery voltage with temperature and discharge rate as parameters, and determine remaining battery capacity by detecting when discharging battery voltage reaches the specified minimum voltage. This method stores minimum voltage as an nth order function of temperature and discharge rate where n is 2 or greater, computes minimum voltage based on the nth order function in memory with temperature and discharge rate as parameters, and assumes remaining battery capacity has reached a set capacity when battery voltage drops to the computed minimum voltage
In this method of calculating remaining battery capacity, the minimum voltage can be set to the battery voltage at 0 to 10% remaining battery capacity.
The method of calculating remaining battery capacity described above is characterized by allowing accurate determination of remaining capacity while reducing the number of constants stored in memory. This is because this method of calculating remaining battery capacity stores discharge efficiency and minimum voltage as nth order functions of discharge rate where n is 2 or greater, computes discharge efficiency and minimum voltage from discharge rate based on the nth order function in memory, and determines remaining battery capacity from the computed discharge efficiency or minimum voltage. For example, in the prior art method shown in FIGS. 1 and 2, 60 constants must be stored in memory to determine discharge efficiency or minimum voltage from discharge rate. On the contrary, in the method of the present invention shown in FIGS. 4 and 6, if discharge efficiency and minimum voltage are computed from 3rd order functions, four constants are needed for each temperature. Therefore, discharge efficiency or minimum voltage can be computed by storing 24 constants for six temperatures. Furthermore, unlike prior art methods which calculate based on a plurality of line segments, the method described above does not have discontinuitles at inflection points. This method can determine discharge efficiency and minimum voltage with greater overall precision and determine remaining battery capacity with little error.